The present invention relates to a mobile unit for dewatering water-containing sludge from a separate sewage system or the like. More specifically, the invention relates to a mobile unit comprising devices for sucking sludge and water from the well, filtering and separating sludge and water and returning purified water, which has been oxygenated in the process, to the well after completion of the process, wherein the separated sludge is compressed and stored in the mobile unit from subsequent emptying. Thus, the invention also relates to a method for dewatering water-containing sludge in a way that helps to return the bacterial or micro flora of the well to an environment that supports an aerobic purification process, made possible with the design of the mobile dewatering unit.
Devices of similar kind are known in the prior art, see e.g. European Patent No. 437,465.
In purification of sludge from septic tanks of the so-called three-compartment type, the entire content of the well is normally transported to a local sewage treatment works or sludge dump. Apart from the disadvantage of disturbing the biological decomposition in the well, the procedure also brings the external sewage sludge to cause a disadvantageous and varying load in the receiving purification plant. This works best where there is an even and regulated supply, while the intermittent load from external sludge runs the risk of clogging strainers and filters and involves high stresses on the biological decomposition in the purification plant, the so called bio-step. Attempts to level out the load on the purification plant comprise deposition in a pool with controlled supply to the biological treatment through pumps. However, this would induce high initial costs, space requirements and problems with smell.
It is an object of the invention to offer a solution to this problem by treating and dewatering the sludge on site, i.e. by the three-compartment well, the septic tank or the like. Apart from the considerably lower load in the purification plant, the advantages of heavily reduced transportation are achieved, at the same time as the biological decomposition in the well is promoted by the addition of oxygenated and purified water.
With the solution according to the invention the well, after purification and separation of sludge, is refilled with its own water containing remaining particles of a certain size for supporting the restored bacterial flora, which is also an objective of the present invention.
The environmental profits directly demonstrable by the device and method according to the invention are the following:
The purification takes place directly by the source and only sucked sludge water in the range of 3-7% has to be dealt with in a subsequent process in a purification plant;
No discharge travels to purification plant or other location has to be made in the meanwhile, but the operator may continuously during the entire work shift dewater wells in the local area and at the end of the work shift empty, or in other suitable way, remove the compressed sludge;
Fewer operational disturbances due to intermittent loads in the purification plant;
Reduced sludge amounts from the purification plant;
The micro-flora in the well is maintained and gets better conditions for its work;
Studies have shown that discharging of suspending substances to infiltration plant or other receiver may be heavily reduced in the order of up to about 75%;
Lower discharge of nitrogen and phosphorus into ground and receivers;
No foreign chemical additives such as polymers are necessary;
The device comprises a possibility to add, during the dewatering process, deposit chemicals for reduction of e.g. phosphorus;
The dewatered sludge with a dry substance of about 25-35% is suitable for energy production in biogas plants;
The sludge can be spread on arable land, as it is free from environmentally interfering chemical additives;
By using a mobile dewatering unit according to the invention there is obtained an improved control of the sludge and a smaller risk of polluting during transport distances;
In addition, the need for transportation and the petrol consumption are reduced since the dewatering unit only has to be emptied at the end of the work shift, i.e. after the working day;
The device lacks pressure vessel, which reduces the overall weight of the unit;
The transported volume can, with the device according to the invention, be reduced to in the order of 5% of the volume transported in conventional sludge emptying.